Data communication method

ABSTRACT

A flag and a method for indicating a predetermined condition in a data communications environment, such as commencement of a transmission, are disclosed. The flag has the property that self correlation of the flag provides an outcome equal to a bit length of the flag and of a first polarity when aligned, and an outcome of zero or of an opposite polarity at any unaligned position.

TECHNICAL FIELD

[0001] The present invention relates to data communications, and in particular to a preamble or flag, and a method of providing a preamble or flag, for use in data communications.

BACKGROUND TO THE INVENTION

[0002] In data transmission systems, there is a need for a receiver to be able to detect when an incoming signal is arriving. Some transmission schemes may use a single start bit, while other schemes may use a longer preamble. In some transmission schemes, flags may be passed from a transmitter to a receiver for other purposes. These schemes can be susceptible to noise, which for example may be present on the transmission medium over which the signal is passing, which may corrupt the start bit, preamble or flag.

[0003] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

[0004] Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

SUMMARY OF THE INVENTION

[0005] It has been realised that a subset of binary sequences only of certain lengths have the property that self correlation of such sequences provides an outcome equal to a bit length of the preamble and of a first polarity when aligned, and an outcome of zero or of an opposite polarity at each unaligned position.

[0006] In accordance with a first aspect, the present invention provides a flag for indicating a predetermined condition in a data communications environment, the flag comprising a binary signal having the property that self correlation of the flag provides an outcome equal to a bit length of the flag and of a first polarity when aligned, and an outcome of zero or of an opposite polarity at any unaligned position.

[0007] According to a second aspect, the present invention provides a method of indicating a predetermined condition in a data communications environment, comprising the step of transmitting a flag having the property that self correlation of the flag provides an outcome equal to a bit length of the flag and of a first polarity when aligned, and an outcome of zero or of an opposite polarity at any unaligned position.

[0008] According to a third aspect, the present invention provides a method of detecting a predetermined condition in a data communications environment, comprising the step of self correlating a flag of an incoming signal and determining that the predetermined condition exists when an outcome of the self correlation is equal to or exceeds a threshold which has a magnitude which is a significant proportion of an expected symbol amplitude multiplied by a bit length of the flag, and is of a first polarity, after all preceding outcomes of the self correlation calculated at unaligned positions have been substantially zero or of an opposite polarity.

[0009] The above noted property of such sequences is particularly useful, as the outcomes produced during self correlation of such sequences give a distinctive indication of when the sequence has been detected, not only through a significantly larger magnitude outcome, but that that outcome is of an opposite polarity to any previous non-zero outcomes of the self correlation. Thus, by use of an appropriate threshold detection technique, such a preamble can be distinguished even in noisy environments. For example, in a noisy environment, an appropriate magnitude threshold may be chosen so as to minimise the probability of both (a) incoming noise falsely causing a determination that a preamble has been received; and (b) the occurrence that a transmitted preamble does not cause a determination that a preamble has been received. However, other magnitudes of the threshold may function in an acceptable manner in some applications.

[0010] For instance, analysis of binary sequences from 2 bits in length up to 16 bits in length has revealed that such sequences exist only for 2 bit, 3 bit, 7 bit, and 11 bit binary sequences. It will be appreciated that a simple algorithm can be used to determine bit sequences of longer lengths which possess this property, and such bit sequences are within the scope of the present invention.

[0011] As an example, the only 7 bit binary sequences which possess this property are 13, 39, 88 and 114 (in decimal); ie 0001101, 0100111, 1011000 and 1110010 (in binary). The 2 bit and 3 bit sequences to possess this property are 01, 10, 001, 011, 100 and 110. It is to be noted that in practice, when calculating the self correlation of the bit sequence or preamble, each zero in the bit sequence is considered to be a −1, hence the outcome of correlation results of negative polarity. The use of NRZ encoded signals may be appropriate in this regard. Further, it is noted that each of the above bit sequences when represented in binary can be seen to be the one's complement of one of the other sequences of equal length.

[0012] The subset of 11 bit sequences which possesses this property consists of 237, 583, 1464 and 1810 (in decimal). It will be appreciated that for such sequences, the self correlation will produce an outcome of zero or less for 10 successive bits, until at an aligned position the outcome will be 11, thereby providing a distinctive pattern which can be easily recognised by receivers or the like. Even if the distinctive pattern is somewhat corrupted due to the presence of noise in the received bit sequence, use of an appropriate threshold decision making circuit may still enable a correct determination as to whether or not the bit sequence has been received.

[0013] Further, in communications environments where a plurality of transmitters use the medium, it is possible that each transmitter may use a different symbol amplitude. The use of preambles which are not in accordance with the present invention and have non-zero and positive self-correlation outcomes at unaligned positions, could lead to an unaligned self correlation result of a large amplitude symbol transmitter exceeding the threshold, and therefore being misinterpreted as an aligned preamble. Consequently, the receiver would be out of phase with the incoming data. However, use of a preamble or flag in accordance with the present invention ensures that, disregarding significant noise, there are no unaligned self-correlation results which are non-zero and positive. Therefore, even where a transmitter uses a relatively large symbol magnitude and the threshold has been determined based on a relatively small symbol amplitude, the self-correlation will not exceed the threshold at unaligned positions, as all unaligned self-correlation outcomes are either zero or of an opposite polarity to the threshold.

[0014] The predetermined condition to be indicated by the flag may be commencement of a transmission, conclusion of a transmission, data packet information, or other type of condition. Hence, it will be appreciated that the term ‘flag’ used in the preceding description encompasses preambles, postambles, mid-packet flags and the like.

[0015] The data communications environment may comprise communication of digital signals in a digital medium, or may comprise communication of digital signals in an analogue medium. It is anticipated that the present invention will be of particular use where the data communications environment comprises communication of digital signals in an analogue medium, such as is used for AC power line communications, wireless data communications, and other analogue carrier environments. In such data communications environments, the signal to noise ratio may be relatively low in the analogue medium. Consequently, bits of the data being communicated may be somewhat or entirely corrupted by noise, and even when no signal is being transmitted, random noise in the medium may be interpreted as data by a demodulator or receiver. While schemes such as those using a single start bit may perform poorly in such environments, use of a flag in accordance with the present invention may provide improved performance.

[0016] It is further noted that the specific examples of flags provided herein in accordance with the present invention have, at most, only one more ‘1’ than ‘0’, or only one more ‘0’ than ‘1’. Accordingly, the use of such preambles reduces a receiver's sensitivity to offset errors. That is, while the aligned correlation result has a magnitude equal to the preamble length multiplied by the symbol amplitude, the correlation result offset error is only equal to the signal offset error for odd length preambles or is zero for even length preambles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings in which:

[0018]FIGS. 1a and 1 b illustrate the use of a preamble in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019]FIG. 1 illustrates an embodiment of the present invention, in which a transmitter 60 communicates with a receiver 61 over a noisy channel 62. Transmitter 60 sends a message m(t) having a preamble 63 which is recognisable to the receiver 61. The preamble 63 has the property that self correlation of the preamble 63 at the receiver 61 provides an outcome equal to a bit length of the preamble and of a first polarity when aligned, and an outcome of zero or of an opposite polarity at any unaligned position. As shown in FIG. 1b, receiver 61 correlates a received signal with the preamble 63, which in the embodiment shown is a 7 bit sequence 0001101. Successive outcomes of the self correlation of the preamble 63 are −1, 0, −1, 0, −1, 0, 7.

[0020] Clearly, the outcomes produced during self correlation of the preamble give a distinctive indication of when the commencement of a transmission has been detected, not only through a significantly larger magnitude outcome, but that all previous outcomes of the self correlation are zero or of an opposite polarity.

[0021] It will be appreciated that in noisy communications environments, a receiver or demodulator will occasionally interpret random noise as data. In prior art schemes such as schemes using a single start bit, such noise-generated data will relatively often be interpreted to indicate a predetermined condition such as commencement of a transmission. However, by use of a particular preamble in accordance with the present invention, the likelihood of noise corruption is significantly reduced compared to such prior art schemes.

[0022] Further, to compensate for noise at the time the preamble is received, a threshold which is a significant fraction of the expected maximum self correlation result may be used to determine whether or not the flag or preamble has been received. Therefore, while some corruption of the incoming data may lead to successive outcomes of the self correlation which differ from the ideal outcomes noted above, the large difference between the non-aligned self correlation outcomes and the aligned self correlation outcome means that it will take a very large amount of noise to cause a false determination of whether or not the flag has been received. Effectively, the signal to noise ratio of the incoming signal is reduced by the square root of the number of bits of the preamble.

[0023] The threshold should be low enough to enable detection of the lowest expected modulation level (symbol amplitude) of any incoming signal, but should be sufficiently large to reduce sensitivity to noise. This can be calculated using standard statistical techniques assuming a normal distribution.

[0024] For example, where a power supply system comprises a plurality n of AC power sources, each of which communicate to a monitoring unit using AC current modulation at potentially different amplitudes, in accordance with the present invention the detection threshold T may be calculated to be:

T=I _(total) ÷n÷I _(mod) ×PS _(detrat)

[0025] where:

[0026] T is the detection threshold;

[0027] I_(total) is the total AC current received from the power supply system;

[0028] n is the number of AC power sources in the power supply system;

[0029] I_(mod) is the ratio of the normal or average current to the modulation current from each power source; and

[0030] PS_(detrat) is the power source detection ratio, to allow for detection of communications from power sources which are operating at only a fraction of the average power source power level.

[0031] Preferably, the threshold should not be allowed to take a value below a certain level, to ensure that noise will not cause a false determination that a flag or preamble has been received.

[0032] Additionally, based on the substantially equal number of ones and zeros in the preamble, the receiver may have the opportunity to compensate for any existing DC offset.

[0033] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. 

1. A flag for indicating a predetermined condition in a data communications environment, the flag comprising a binary signal having the property that self correlation of the flag provides an outcome equal to a bit length of the flag and of a first polarity when aligned, and an outcome of zero or of an opposite polarity at any unaligned position.
 2. The flag of claim 1, wherein the flag has a bit length chosen from the group of two bits, three bits, seven bits and eleven bits.
 3. The flag of claim 1 or claim 2, wherein the predetermined condition comprises commencement of a transmission.
 4. The flag of claim 1 or claim 2, wherein the predetermined condition comprises conclusion of a transmission.
 5. The flag of claim 1 or claim 2, wherein the predetermined condition comprises a mid-packet flag condition.
 6. The flag of any preceding claim wherein the data communications environment comprises communication of digital signals in an analogue medium.
 7. The flag of any one of claims 1 to 5 wherein the data communications environment comprises communication of digital signals in a digital medium.
 8. The flag of claim 6 wherein the data communications environment comprises AC power line communications.
 9. The flag of claim 6 wherein the data communications environment comprises wireless data communications.
 10. A method of indicating a predetermined condition in a data communications environment, comprising the step of transmitting a flag having the property that self correlation of the flag provides an outcome equal to a bit length of the flag and of a first polarity when aligned, and an outcome of zero or of an opposite polarity at any unaligned position.
 11. The method of claim 10, wherein the flag has a bit length chosen from the group of two bits, three bits, seven bits and eleven bits.
 12. The method of claim 10 or claim 11, wherein the predetermined condition comprises commencement of a transmission.
 13. The method of claim 10 or claim 11, wherein the predetermined condition comprises conclusion of a transmission.
 14. The method of claim 10 or claim 11, wherein the predetermined condition comprises a mid-packet flag condition.
 15. The method of any one of claims 10 to 14 wherein the data communications environment comprises communication of digital signals in an analogue medium.
 16. The method of any one of claims 10 to 14 wherein the data communications environment comprises communication of digital signals in a digital medium.
 17. The method of claim 15 wherein the data communications environment comprises AC power line communications.
 18. The method of claim 15 wherein the data communications environment comprises wireless data communications.
 19. A method of detecting a predetermined condition in a data communications environment, comprising the step of self correlating a flag of an incoming signal and determining that the predetermined condition exists when an outcome of the self correlation is equal to is equal to or exceeds a threshold which has a magnitude which is a significant proportion of an expected symbol amplitude multiplied by a bit length of the flag and is of a first polarity, after all preceding outcomes of the self correlation calculated at unaligned positions have been substantially zero or of an opposite polarity.
 20. The method of claim 19, wherein the flag has a bit length chosen from the group of two bits, three bits, seven bits and eleven bits.
 21. The method of claim 19 or claim 20, wherein the predetermined condition comprises commencement of a transmission.
 22. The method of claim 19 or claim 20, wherein the predetermined condition comprises conclusion of a transmission.
 23. The method of claim 19 or claim 20, wherein the predetermined condition comprises a mid-packet flag condition.
 24. The method of any one of claims 19 to 23 wherein the data communications environment comprises communication of digital signals in an analogue medium.
 25. The method of any one of claims 19 to 23 wherein the data communications environment comprises communication of digital signals in a digital medium.
 26. The method of claim 24 wherein the data communications environment comprises AC power line communications.
 27. The method of claim 24 wherein the data communications environment comprises wireless data communications.
 28. The method of any one of claims 19 to 27 wherein the threshold is chosen to be substantially equal to the bit length of the preamble in magnitude when the data communications environment is substantially noise free.
 29. The method of any one of claims 19 to 27 wherein, when the data communications environment is relatively noisy, the threshold magnitude is chosen so as to minimise the probability of both (a) incoming noise falsely causing a determination that a preamble has been received; and (b) the occurrence that a transmitted preamble does not cause a determination that a preamble has been received. 